The invention relates to an apparatus for producing an article from a thermoplastic or thermosetting material by using a molding technique. It also relates to a method for producing such an article and to a method for packaging semiconductor devices.
Molding techniques or injection-molding methods are widely used for processing plastics. With them, many different articles can be produced, for example from thermoplastic or thermosetting materials. The techniques of transfer molding and compression molding are used particularly often.
In the case of compression molding, the molding material, which may for example take the form of a paste or a quantity of pellets and is generally preheated, is introduced into an open, usually heated mold cavity, which is subsequently closed. The molding material is pressed into the mold by the effect of a pressure and fills it completely. The molding material remains in the mold until it has completely cured.
In the case of transfer molding, a defined quantity of the molding material, typically a thermosetting material, is filled into the otherwise closed mold cavity through a gate. During transfer molding, the walls of the mold are typically heated to a temperature above the melting temperature of the molding material in order to obtain good flow properties of the material within the cavity.
A large number of different thermoplastic and thermosetting materials come into consideration both for compression molding and for transfer molding.
It is possible both in the case of compression molding and in the case of transfer molding to produce a large number of usually identical articles simultaneously if the mold has more than one mold cavity, it also being possible for a number of mold cavities to be connected to one another by narrow openings in which fins form during molding. In the case of transfer molding, the molding material is then usually introduced into the individual cavities simultaneously from the supply container through a branched feed line (sprue and runner).
In particular when thermosetting materials are used, which as a result of the contact with the hot mold crosslink to an increasing extent as they flow over an increasing distance, or highly viscous thermoplastic materials are used, in particular with a mold that is not heated, the formation of voids (pockets of air or gas resembling bubbles or cracks) can cause flaws in the articles produced. Voids form in particular in regions within the mold cavity that are of a particularly intricate and capillary nature, and in cases where particularly small articles are produced. Voids may represent blemishes, but may also lead to malfunctions of the article produced, for example in the case of semiconductor devices.
Various efforts are therefore made to prevent the formation of voids. For example, in the production of plastic packages of semiconductor devices by transfer molding, the air present in the mold cavity is removed through vents during the molding process. A negative pressure of approximately 100 mbar is thereby reached in the mold cavity. It is particularly favorable in this case if the vents are arranged in the wall of the mold cavity that lies opposite the gate, because in this case the air can be removed from the entire flow path of the molding material. The vents are typically in gaps between the two parts of the mold.
This vacuum molding does improve the result of the molding process and reduces the probability of voids forming. However, in return it must be accepted that the molding compound penetrates into the vents and unwelcome bleed edges form as a result on the article produced.
For these and other reasons, there is a need for the present invention.